Black frogs of Chernobyl.  Expression of evolution in the exclusion zone

Environmental pollution is ubiquitous, but its long-term environmental consequences are not always clear and are still poorly studied. This is the case, for example, in the case of radioactive contamination after a serious accident at the Chernobyl nuclear power plant. To date, very little research has been done on the evolution of organisms chronically exposed to ionizing radiation, as in the exclusion zone around the Chernobyl nuclear power plant.

The accident at the fourth reactor of the Chernobyl nuclear power plant in 1986 resulted in the largest release of radioactive materials into the environment in history. The environmental impact of acute exposure to high doses of radiation was severe. However, more than three decades after the accident, Chernobyl has become one of the largest nature reserves in Europe. Today, a variety of endangered species find refuge here, including bears, wolves, and lynxes.

Ionizing radiation can destroy the genetic material of living organisms and generate unwanted mutations. However, one of the most interesting research topics in the Chernobyl region is trying to find out if there are any species that have adapted to life in such an environment. As with other pollutants, ionizing radiation can be a very strong selective factor that favors organisms with mechanisms that increase their adaptability in areas contaminated with radioactive materials.

German Orizola from the University of Oviedo and Pablo Buraco from Estación Biológica de Doñana came to Chernobyl in 2016. Then they found several oriental frogs near the damaged nuclear reactor (hello orientales) in an unusual black color. These species usually have a light green color, although darker specimens can sometimes be found.

Black frogs prompted scientists to investigate the role of skin pigmentation among Chernobyl’s wife frogs. In 2017-2019, they studied tree frog coloration in detail in different regions of northern Ukraine. These sites are located along the Chernobyl zone, where there are varying levels of radioactive contamination. These included some of the most radioactive regions on the planet, but also sites outside the exclusion zone with low levels of radiation used as a control in this study.

During these three years, Orizola and Buraku analyzed the skin color of more than 200 male oriental frogs. In addition to skin color, they also examined the radiation dose and oxidative stress to which the amphibians were exposed. It turned out that most of the frogs caught outside the protected area were light green – a color typical of this species. However, the samples collected in the exclusion zone were darker, some black.

After more detailed analyzes, which appeared in the pages of the magazine “Evolutionary Applications” (DOI: 10.1111 / eva.13476), it turned out that the dark color is typical of frogs caught in the most polluted areas at the time of the accident. . The color was not correlated with the levels of radiation that amphibians are currently exposed to, which the researchers measured in all individuals.

From the start, researchers suspected that exposure to high doses of ionizing radiation contributes to dark skin color. It is known that the dark color protects from various sources of radiation and in many organisms it is melanin that is responsible for this. This pigment is responsible for the color of the skin and eyes, in human hair and in animals’ fur. But melanin can also reduce the negative effects of UV rays.

but that is not all. Melanin can also protect against ionizing radiation, as shown with fungi. In studies two years ago, it was demonstrated that fungi in a destroyed nuclear reactor at the Chernobyl plant use melanin to absorb radiation and convert it into chemical energy they need for growth, possibly in a similar way to how plants use green dye. Chlorophyll for energy from photosynthesis (more on this in the text: Mushrooms from the Chernobyl reactor could be used as a radiation shield in space).

The melanin absorbs and diffuses some of the radiation energy. In addition, it can remove and neutralize ionized molecules inside the cell such as reactive oxygen species. These actions reduce the potential for cell damage in organisms exposed to radiation and increase the survival chances of those organisms.

Melanin pigmentation has been proposed as a buffer mechanism against ionizing radiation. The research findings suggest that exposure to high levels of ionizing radiation, likely during and shortly after the accident, may have resulted in a darker coloration of the eastern Chernobyl tree frog. However, the scientists stress that more research is needed to determine the underlying mechanisms and evolutionary consequences of the patterns found here.

As the researchers acknowledge in The Conversation, the results of their analysis suggest that the Chernobyl frogs may have undergone a rapid evolution process in response to ionizing radiation. In this scenario, the frogs that were darker in color at the time of plant failure, and who are usually a minority in their populations, were protected by melanin.

These individuals adapted better to radiation and continued to reproduce. More than 30 years have passed since the disaster. During this time, the darker specimens survived for several generations, and a classic, albeit very rapid, process of natural selection may explain why dark frogs are now the dominant species in the Chernobyl region.

The study of Chernobyl black frogs is the first step to better understanding the protective role of melanin in environments affected by radioactive contamination. In addition, it opens the door to promising applications in fields as diverse as nuclear waste management and space exploration.

“We hope that the war in Ukraine will end soon and the international scientific community will be able to return to studying the remarkable evolutionary processes at Chernobyl with our Ukrainian colleagues,” the researchers wrote.

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